1. Field of the Invention
This invention pertains to municipal solid waste combustors (MWCs) and, by extension, to all solid fuel steam power generation boilers and other furnaces. It involves odor free handling and disposal by thermal decomposition of waste sludges of variable solids content, including municipal sewage sludge, paper mill sludge, and other industrial sludges, in MWC power boilers and other furnaces which burn municipal solid waste or other solid fuels.
2. Background of the Related Art
As the options available for communities and industries for sludge disposal decline, a new cost-effective, environmentally sound method of disposal has become desperately needed. In the 1950s and 1960s, sludges were often incinerated in multiple hearth furnaces. However, most of those installations experienced poor combustion and produced extremely adverse air emissions. Accordingly, this approach was largely abandoned in favor of non-combustion disposal technologies.
Co-combustion of sludge, and preferably high carbon content sludge, with other fuels has remained a goal for many years. One approach, discussed repeatedly in technical literature, was to design external thermal drying systems for pre-drying sludge prior to injection into a furnace, so that drying sludge inside the furnace would not be necessary. However, this technology was expensive and even dangerous, because of potential health hazards related to inhalation of fugitive dust and dust explosions.
Other approaches described below involved injection of wet sludge into modern combustion processes. These approaches failed to achieve commercial acceptance of their technologies because of limited applicability, economic disadvantages or technological disadvantages. Additionally, conventional approaches did not address the problems of controlling odor in the handling and transporting of sludge prior to introduction into a furnace.
Komline et al. in U.S. Pat. No. 3,322,079 noted the synergy in co-combustion of sludge and municipal solid waste (MSW), using high temperature gases from combustion of MSW to dry sludge injected into the furnace by a rotary centrifugal atomizer. However, operating costs of such a system are high and the rotary centrifugal atomizer disclosed is difficult to reliably maintain because of plugging and wear. The rotary atomizer discharges a significant fraction of large particles that do not burn completely, and deposition of material on boiler internals becomes a problem.
Dingwell in U.S. Pat. No. 3,838,651 used steam to atomize waste oil in a burner designed to extend into a combustion chamber. However, this invention is limited in scope to waste oils in old-fashioned incinerators, which were not regulated as stringently at the time of invention. Partial combustion was acceptable and regulatory compliance was not identified as a goal. While steam was used for atomization, non-oil, i.e. aqueous, sludges were not considered.
Pan in U.S. Pat. No. 3,903,813 developed a device for injecting steam-atomized sludge into a combustion chamber in close proximity to an oil or gas burner. The injection device allows for mixing of a pressurized fluid and sludge in the same pipe, with the mixture exiting a restrictive orifice at the end of the injection pipe, such that the mixture is flashed to atmospheric pressure inside the furnace in the presence of oil or gas. Co-combustion with MSW and other renewable and fossil fuels is not considered and the device is intended for use primarily with sludges having solids content equal to or less than 5%. The point of sludge injection is limited to an area below an oil or gas burner flame. This device is intended for incineration of sludge only in the presence of an oil or gas burner.
Carpenter in U.S. Pat. No. 5,284,405 disclosed a method for entraining sludge particles in a stream of compressed combustion air, but the invention is limited to combustion within a rotary cement kiln. No attempt is made to burn sludge in suspension and regulatory compliance is not addressed.
Goff et al. in U.S. Pat. Nos. 5,052,310 and 5,405,537 established a sludge injection system for MWC""s using oxygen-enriched air both to atomize sludge being sprayed into a furnace through a nozzle and to offset the loss of boiler efficiency from moisture in the sludge. However, this requires expensive construction of an oxygen-producing plant adjacent to an owner""s core production facilities or purchase of large quantities of bottled oxygen. Furthermore, the invention is limited to MWC""s only and may require modification to existing combustion air control systems.
Mole in U.S. Pat. No. 5,531,169 injects liquid waste material, primarily contaminated acid, immediately adjacent to a primary fuel burner. The primary purpose of the device is to dissociate acid molecules. The device is intended for air as opposed to steam atomization and the invention is directed to liquid hazardous waste disposal as opposed to sludge disposal and non-hazardous municipal solid waste combustion and conventional power generation.
Guibelin in U.S. Pat. No. 5,544,598 developed a nozzle for disposal of slurry, such as pasty or fatty wastes, by spraying onto burning municipal solid waste. However, this device does not involve sludge, and it is not designed for atomizing particles to burn in suspension.
An objective of the present invention is to provide a sludge receiving and treatment system that is capable of handling sludges with a wide range of characteristics (including variable moisture and solids content) and delivering a consistent flow of sludge to the injection system. Previous systems were not designed for and did not address variable sludge consistencies and solids content.
It is also a goal of the present invention to avoid the material handling difficulties typically encountered with sludge having solids contents of 15% and higher.
A further purpose of the present invention is to maximize suspension burning of sludge particles, in order to virtually eliminate the need to burn larger sludge particles.
A still further goal of the present invention is to utilize wastewater, other industrial process liquid wastes, air pollution control slurries, or mixtures thereof, in a sludge pre-combustion treatment process.
Another goal of the present invention is to eliminate odor in the handling, treatment and burning of sludge.
The aforementioned purposes, goals and objectives are achieved by the present invention for treating and injecting sludge into a combustor, which substantially reduces or eliminates the shortcomings that limited commercial viability of prior techniques.
The present invention generally includes a sludge receiving and treatment module, a solids controller and a sludge injection and combustion module. The sludge receiving and treatment module receives and treats sludge by either adding a liquid selected from the group consisting of water, aqueous based solutions, plant wastewater, industrial process liquid waste, pollution control slurries and mixtures thereof, or by reducing the moisture content of said sludge, to obtain a sludge having a desired moisture content. The solids controller monitors and varies the moisture content of the treated sludge, while the sludge injection and combustion module injects the treated sludge into a combustion zone of a combustor having a primary solid fuel burning therein, wherein the treated sludge is burned in suspension within the combustion zone of the combustor. The combustor can be a municipal solid waste combustor, having municipal solid waste burning therein, or it can be any solid fuel steam power generation combustor.
Preferably, the sludge injection and combustion module includes an injection nozzle for atomizing and preheating the treated sludge with steam and spraying the atomized treated sludge into the combustion zone of the combustor. The injection nozzle preferably includes a discharge end configured to provide a helical shaped discharge of sludge from the nozzle.
The system further preferably includes a controller for monitoring the temperature within the combustor and for varying the amount of treated sludge injected into the combustor in response to temperature changes within the combustor. The sludge receiving and treatment module further preferably includes a hopper for receiving and storing the sludge and a mixer for receiving the sludge from the hopper and for receiving liquid from a liquid source. The sludge injection and combustion module further preferably includes a sludge tank for receiving and storing the treated sludge from the sludge receiving and treatment module and a pump in fluid communication with the sludge tank and the injection nozzle for delivering the sludge from the sludge tank to the injection nozzle.
The sludge tank preferably includes a level controller for monitoring the level of sludge within the sludge tank and generating a signal to the sludge receiving and treatment module to vary the amount of sludge received in the sludge tank to maintain a constant level of sludge within the sludge tank, a mixer for continuously agitating the sludge within the sludge tank, a discharge line for delivering the sludge from the sludge tank to the pump and a conditioner interposed in the discharge line for shredding the sludge before entering said pump. A recirculation line is preferably connected between the pump and the sludge tank for recirculating sludge back to the sludge tank and a recirculation control valve is connected to the recirculation line for controlling a flow of sludge through the recirculation line. A flow controller is also preferably connected to the pump for monitoring the flow of sludge through the pump and generating a signal to the recirculation control valve to vary the amount of sludge flowing through the recirculation line.
In a method for co-combusting sludge within a combustor according to the present invention, a received sludge is first treated either by adding a liquid selected from the group consisting of water, aqueous based solutions, plant wastewater, industrial process liquid waste, pollution control slurries and mixtures thereof, or by reducing the moisture content of the received sludge, to obtain a sludge having a desired moisture content. The solids content of the treated sludge is monitored utilizing a solids controller and the moisture content is varied in response to solids content changes of the treated sludge. The sludge is then injected into a combustion zone of a combustor having a primary solid fuel burning therein where it is burned in suspension within the combustion zone of the combustor.
Preferably, the method further includes the steps of atomizing the treated sludge with steam prior to injection into the combustion zone, monitoring the temperature within the combustor and varying the amount of treated sludge injected into the combustor in response to temperature changes within the combustor to control combustor temperature.
The present invention further provides a sludge handling system having improved odor control. The system generally includes a sludge receiving storage tank, which receives sludge by pipeline or tanker trailer, and a sludge injection and combustion module for injecting the sludge from the storage tank into a combustion zone of a combustor. The sludge receiving storage tank includes an air diffuser for mixing and aerating the sludge within the tank for odor control. The storage tank further preferably includes a vent header having a filter system for controlling odors from the air vented from the vent header.
For further improvement of odor control when the sludge is delivered by tanker trailer, the present invention includes a sludge tanker trailer having an air diffuser for mixing and aerating sludge within the trailer. The sludge tanker trailer further preferably includes a vent header having a filter system for controlling odors from the air vented from the sludge tanker trailer by the vent header. The sludge tanker trailer also preferably includes a level transmitter for indicating the level of sludge within the trailer and an air blower for providing fresh air into the trailer thereby providing a positive air pressure within the trailer for pneumatic unloading of sludge. A diffuser header is also preferably connected to the air diffuser and a source of tractor engine exhaust is connected to the diffuser header for directing the tractor engine exhaust into the sludge tanker trailer through the air diffuser.
The steam atomization nozzle utilized in the present invention is specially engineered to allow flash drying and maximum particle size reduction upon contact with the atomizing steam prior to injection into the furnace. This allows for near complete combustion, little or no impact on bottom ash quality and burnout, and treatment of virtually all products of sludge combustion in existing air pollution control systems. Previous inventions did not address the goal of suspension burning or compliance with air pollution regulations.
The present invention is designed to allow delivery, modification, and control of solids content both to maximize the number of potential sludge customers and to stabilize furnace temperature in response to fluctuating primary fuel heating value.
For MWCs, MSW heating value fluctuates seasonally and hour-to-hour, depending on the source of solid waste deliveries. The present invention allows for manual adjustment of sludge characteristics, including solids and carbon content, and flow rate to maintain relatively constant temperature in the furnace resulting in smoother steam generation. Moreover, because the average heating value of MSW has increased significantly since many MWC""s were designed, solid waste through-put has been limited by the higher heating value of the fuel. Since solid waste is a major source of revenue for a MWC, reduced through-put capacity can result in reduced revenue. The disclosed method allows for maintaining MWC through-put revenues at higher than design MSW heating value.
For all industrial combustion facilities, the present invention can dispose of plant wastewater and other industrial process liquid wastes, as well as reduce overall air pollution control costs.
For industrial sludge producers such as paper mills and other similar installations, the present invention makes it possible to achieve environmentally sound on-site or local sludge disposal using existing conventional solid fuel steam boilers, as opposed to continuously increasing sludge hauling and disposal costs to controlled landfills.
The preferred embodiments of the sludge treatment and injection system, as well as other objects, features and advantages of this invention, will be apparent from the following detailed description, which is to be read in conjunction with the accompanying drawings.